1. Field of the Invention
The present invention relates generally to a network system which enables data sharing using a client/server architecture and, more particularly, pertains to terminals connected to such a network system.
2. Description of the Related Art
In the marine electronics industry, a network system which permits sharing of such data as destination data and route data among various onboard apparatuses connected to the network system is conventionally known. The apparatuses connected to the network system may include a course plotter, a Global Positioning System (GPS) receiver, a radar and an echo sounder, for example. A client/server system is one form of this kind of network system, in which a server manages shared data which will be provided to a client on request.
Japanese Patent Application Publication No. 1991-206535 introduces an example of a data sharing method usable in a client/server system. The client/server system disclosed in this Publication is a generally applicable network system which is not limited to networking of apparatuses installed on a vessel. The Publication describes an information processing system including a file server for storing and managing shared files and a plurality of user terminals (clients) which are interconnected by a network, wherein the shared files are managed in a below-described manner. Specifically, the individual user terminals are provided with storage devices for storing files necessary for themselves whereas the file server is provided with file management means. Among the shared files stored in the file server, files required by the individual user terminals are transferred to the pertinent user terminals and stored in the storage devices thereof at first and, thereafter, the file management means of the file server manages the shared files in such a way that the files stored in the storage devices of the individual user terminals coincide with corresponding portions of the shared files. If any of the user terminals needs to access a particular shared file, the relevant user terminal accesses the file stored in the storage device of the same user terminal. According to the Publication, the client/server system thus configured permits high-speed access to the shared files.
The aforementioned client/server system of the Publication is configured based on the assumption that the file server and the user terminals would permanently serve as a server and clients, respectively. Practically, however, a client/server network may be configured such that each terminal connected to the network can work either as a server or as a client depending on situations. In this kind of client/server system, one terminal selected from a plurality of terminals connected to the network serves as a server while the other terminals work as clients at any given time. One benefit of this kind of client/server system is that if a failure occurs in the server, one of the clients can be selected as a new server, thus permitting uninterrupted data sharing. This client/server architecture makes it possible to create a flexible networking environment.
One approach to realizing the aforementioned flexible networking environment would be to configure the client/server system in such a way that a desired one of the terminals selected by a user by manual operation is set to operate as a server and the other terminals to work as clients, for instance. In this approach, client/server information managed by the individual terminals should include information on the terminal specified as the server and information on the terminals specified as the clients.
In this client/server architecture, the terminals set to work as the clients are allowed to access the terminal set to act as the server which manages data sharing among the individual terminals. In the following discussion of the present invention, the terminal acting as the server is referred to as the “server terminal” and the terminals working as the clients are referred to as the “client terminals.”
In the network system thus configured, if a particular client terminal is powered on when the server terminal is not powered on, the client terminal will not be able to identify any server on the network. Data sharing will be impossible in such a situation, so that the pertinent client terminal presents a warning message to prompt the user to turn on the server terminal or carry out various settings once again.
Also, the network system may be reconfigured by adding a new terminal to the network or replacing an existing terminal with a new one, for example. The server terminal may be reset to work as a client or the server terminal may be powered off in a process of such network reconfiguration. Also, although only one terminal on the network should normally be set to act as the server, more than one terminal on the network might be set to act as servers due to an error in user setting, for instance.
Since the network system can not correctly operate in such situations, the server and client terminals would present warning messages and it is considerably complex and difficult for the user to cope with such warning messages.